With rapid market expansion of portable electronic devices, such as a laptop computer and a cell phone, demands of small-sized high capacity rechargeable batteries with a high energy density and excellent charging and discharging cycle characteristics have been increased for use in threes electric devices. In order to meet such demands, nonaqueous electrolyte rechargeable batteries have been developed. A nonaqueous electrolyte rechargeable battery uses alkali metal ions, such as lithium ions, as a charge carrier, and causes an electrochemical reaction in accordance with reception of charged particles of the charge carrier.
With regard to a nonaqueous electrolyte rechargeable battery used for vehicles, such as electric vehicles, higher durability performance has been required to meet with the expected lifetime of the vehicles. In a nonaqueous electrolyte rechargeable battery, if an irreversible reaction, such as incorporation of lithium into a coating when the coating is formed, advances in addition to a regular battery reaction occurring during charging and discharging, the amount of lithium that can contribute to the battery reaction decreases. The decrease in the amount of lithium results in deterioration of a charging and discharging capacity.
For example, JP2010-102841A describes a nonaqueous electrolyte lithium-ion secondary battery with improved durability. JP2010-102841A aims to minimize the decrease in energy density due to irreversible capacity in initial charging and discharging by using a high-capacity positive electrode. As a positive electrode active material that is consumed by the initial irreversible capacity specific to a negative electrode active material including at least one selected from silicon (Si), silicon oxide (Si) and carbon (C), a higher capacity lithium-containing composite nitride represented by a formula Li3-XMXN (M is one or more transition metals selected from cobalt (Co), nickel (Ni), and copper (Cu), and x is equal to or greater than zero and equal to or less than 0.8) is used. Therefore, the weight of the positive electrode active material that is not used for charging and discharging after the initial discharge is reduced, and hence the energy density of the nonaqueous electrolyte lithium-ion secondary battery increases.
That is, in the secondary battery of JP2010-102841A, the total amount of lithium in the cell is increased by using the high capacity positive electrode. Therefore, even if the lithium is deactivated by the irreversible reaction, the absolute amount of the lithium in the battery is secured, and hence the high capacity is achieved.
In the secondary battery where the source of lithium is merely added in the positive electrode, as JP2010-102841A, the capacity seems to be increased. However, it is difficult to restrict the advance of the irreversible reaction, which is a substantial cause of the capacity deterioration. Although the durability improves for the amount of lithium added, the capacity eventually reduces. Accordingly, the effect is not highly expected in use for a long time.